The invention disclosed and claimed herein is generally directed to apparatus for cooling the oil or other fluid which is used to carry heat away from the X-ray tube in a computed tomography (CT) diagnostic imaging system. More particularly, the invention is directed to apparatus of such type which significantly reduces acoustic noise which is likely to disturb or annoy CT imaging patients and operating personnel.
As is well known in the art, an X-ray tube generates substantial amounts of heat in the course of its operation. Accordingly, provision must be made for removing heat from the proximity of the tube, and for dispersing the heat into the surrounding environment. In a common arrangement, a coolant fluid or oil, such as a product sold by Exxon under the name UNI-Volt, circulates around the tube to receive excessive heat, and then flows through a first hose, conduit, or the like to a heat exchanger. The exchanger causes the heat stored in the oil to be exposed to the surrounding air, so that the heat can be convected thereinto. The cooled oil then flows back to the tube through a second hose or conduit.
Cooling arrangements of the above type commonly employ a fan to move air past or through the heat exchanger, to enhance heat transfer. If the X-ray tube is used in connection with a CT system, the tube, the heat exchanger, and the cooling fan are respectively mounted on an annular gantry, which is rapidly rotated around the patient to acquire a CT image. The gantry may rotate, for example, at 90 rpm. At present, the X-ray tube cooling fans used in CT systems tend to be axial. That is, both the intake and exhaust air streams generated by the fan are directed along the fan axis, i.e., the axis of blade rotation. Herein, "intake" and "exhaust" air streams mean the streams or quantities of air which are respectively moved into and out of a fan by operation thereof. To provide sufficient cooling power, axial cooling fans must be rotated at a speed on the order of 3600 rpm.
In order to resist vibration and provide some measure of sturdiness or rigidity, axial fans generally have a number of struts or like members positioned around the outer edges of the fan blades, in spaced apart relationship. Each time a blade passes by one of the struts, an acoustic noise is produced. For the fan rotational speed stated above, the pure tone, or first harmonic of such noise will exceed 500 Hz. Accordingly, the second and higher harmonics of such noise will exceed 1000 Hz. As is known by those of skill in the art, 1000 Hz is a very significant threshold in reducing the "annoyance factor" associated with acoustic noise. That is, acoustic noise having principal frequency components which exceed 1000 Hz tends to be much more disturbing than noise which does not include such components.
The 1000 Hz threshold and the associated acoustic noise "annoyance factor" are of particular significance in regard to X-ray tube cooling systems for use in CT applications. When a CT system is being operated to acquire an image, the patient or other imaging subject must remain as still as possible, for a period of time. Accordingly, it is desirable to minimize bothersome noises as much as possible, to avoid distracting, irritating, or in some cases, frightening, the patient. Excessive noise may also be disturbing to persons operating the CT system. Higher frequency gantry sound can also interfere with the patient speaker and microphone used in most systems to maintain continuous patient communication.
One approach to relieving noise generated by a prior art cooling fan would be to reduce the rotational speed thereof. Principal frequency components of acoustic noise generated by the fan could thereby be reduced below the 1000 Hz threshold. However, the air flow provided by the fan to the heat exchanger would also be significantly reduced thereby, so that the thermal performance of the X-ray tube cooling system could be significantly diminished.